1. Field of the Invention
This invention relates to electrical connectors formed of wire and each shaped to be securely held by being soldered into a hole in a supporting printed circuit board so that each connector can press against and make good electrical contact with a conductive area on only one surface along the edge of a subsidiary printed circuit board and yet is prevented from shifting position as a result of cold flow of the solidified solder that is supposed to hold the connector stationary. It further relates to connector means comprising pairs of such wire connectors soldered in mirror-image relationship in parallel rows of holes in the supporting board to apply pressure against opposite surfaces of the subsidiary board.
2. The Prior Art
My U.S. Pat. No. 3,340,440 shows several configurations of connectors formed of wire. Each of the connectors has two ends arranged so that they can be inserted in holes in a printed circuit board and soldered to conductive pads on the board immediately around the holes. Each connector consists of one piece of wire bent so that it forms at least one pair of U-shaped loops spaced apart in the plane of the wire connector by a distance that allows one edge of a second, or subsidiary, printed circuit board to be inserted in the gap between the loops. The boards to which the ends of the wire connectors are attached are frequently referred to as "mother boards" and the subsidiary boards inserted between the connector loops are frequently referred to as "daughter boards".
Of all of the connector configurations shown in the aforesaid U.S. Pat. No. 3,340,440, the one that has achieved the greatest commerical success is the one that resembles an "M". That connector has two side wire portions, each of which has a straight section and a U-shaped loop. In each of the U-shaped loops, the wire is curved toward the other loop and is in the same plane as the other loop. The continuous section of wire that joins the two loops and constitutes the central part of the connector is also formed in the shape of a "U", which is open in the opposite direction from the U-shaped loops of the side wire portions. The sides of the central U-shaped portion are tilted or curved slightly toward each other so that juxtaposed contact areas of the two U-shaped loops of the side wire portions can receive an edge of a daughter board and make electrical and mechanical pressure contact with the opposite surfaces of the board at locations spaced slightly away from that edge.
An M-shaped connector of the type just described cannot be used with a type of printed circuit daughter board in which contact areas directly aligned with each other on opposite surfaces are not supposed to be short-circuited together. It frequently is important to provide such a large number of electrically separate connection areas spaced along a certain length of printed circuit board edge that the connection areas on one surface must be electrically isolated from directly opposite connection areas of the opposite surface. Since the edge of the daughter board is grasped by two contact areas of each M-shaped connector, each connection area on one surface of that edge is directly connected by the central part of the connector to a corresponding area on the opposite surface. Thus, the two connection areas on the board are unavoidably short-circuited by the connector, even though it would be desirable for those connection areas to be connected to different circuits on the board.
It would appear that such short-circuiting connection between the two contact areas of an M-shaped connector could be avoided by simply cutting off the central part of the connector, leaving just the side wire portions, each of which would then be J-shaped. Rather than forming the complete M-shaped connector and cutting the central part away, just the J-shaped members would be formed. However, experimentation has shown that such a simple solution does not work. The two side wire portions of a complete M-shaped connector help align each other for insertion in a pair of holes in a mother board, and the two contact areas of the side loops are automatically in confronting relation. While they may require jigs to space them exactly correctly apart, as described in my U.S. Pat. No. 3,940,849, such spacing is made permanent by the solder that solidifies around the straight section of each of the side wire portions. A substantial part of the resilient force that presses the two contact areas of an M-shaped connector firmly against opposite surfaces of a daughter board is provided by the resilience of the central part of the connector, and although there is some outward force on the solder that holds the side wire portions in place in the mother board, such force is not great enough to cause sufficient cold flow of the solder to affect the spacing between the contact areas substantially.
That is not the case with simple J-shaped connectors. The holes into which the straight wire portions of M-shaped connectors are inserted are substantially larger than the connector wire, e.g., about 0.031" for an M-shaped connector made of 0.0201" diameter wire. Attempting to hold J-shaped connectors formed from just the side portions of an M-shaped connector of 0.0201" diameter wire in 0.031" diameter holes resulted in unacceptable shifting of the supposedly rigidly fixed connectors due to cold flow of the solder. The pressure of each contact area against one surface of a daughter board is only about one ounce, but due to the small diameter of the wire, that small contact pressure results in a force of about 2000 p.s.i. on the solder. Solder will cold flow, even at room temperature, at pressures as low as a few hundred p.s.i. or even less than 100 p.s.i. at 80.degree. C. The change in positions of pairs of juxtaposed contact areas of two such J-shaped connectors is so great that the contact pressure on the daughter board falls to an unacceptably low value after only a few days of use.
There is a further disadvantage of simply forming J-shaped connectors shaped identically with the loops and straight sections of side wire portions of the prior art M-shaped connectors. In the M-shaped configuration it is necessary that the central part of the connector be able to flex enough to accommodate daughter boards varying from slightly below the nominal thickness to slightly above it while still applying approximately the correct pressure to the board. This requires that the contact areas be on the confronting surfaces of the U-shaped loops of the side wire portions and as far as the size of the components will permit from the bight of the central portion. If the J-shaped connectors are made with exactly that configuration, the flexible length, which includes the U-shaped loop and the part of the straight section from the loop to the solder is relatively short and makes such a J-shaped member so stiff that it is difficult to position it in the mother board accurately enough to cause it to exert exactly the desired pressure on the daughter board.